1. Field of the Invention
The present invention relates to a magnetic disk cartridge, more particularly to an improved cassette shell for a magnetic disk cartridge having a rotatable magnetic disk encased in a cassette shell.
2. Description of the Related Art
A magnetic disk cartridge comprises a magnetic disk for magnetically recording analog and/or digital signals rotatably encased in a thin cassette shell constituted of a pair of plastic cassette shell halves that are mated and joined together. The cassette shell is formed in the shape of a flat prism (substantially a hexahedron) with an approximately square top surface slightly larger than the magnetic disk, a bottom surface having an opening through which a center core supporting the center of the magnetic disk is exposed and narrow side surfaces extending between the outer peripheral edges of the top and bottom surfaces. The magnetic disk cartridge is provided with a magnetic head access opening for insertion/withdrawal of magnetic heads used for magnetic recording and reproducing so as to enable the magnetic heads to be brought into contact with, or close to, the opposite surfaces of the magnetic disk from the exterior. The magnetic head access opening is equipped with a shutter for preventing invasion of dust and the like when the magnetic disk cartridge is not in use.
Liners made of nonwoven fabric or the like are attached to the inner surfaces of the cassette shell opposite the main surfaces of the magnetic disk for protecting the surfaces of the magnetic disk from scratching as well as for wiping off dust and the like adhering thereto.
In recent years, new high-density recording methods have led to the development of a magnetic disk cartridge with 50 times the capacity of the widely used conventional magnetic disk cartridge (3.5-inch floppy disk). In order to increase the recording density and the data transfer rate of such a high-capacity magnetic disk cartridge, the magnetic disk is rotated at a much faster speed than the conventional one and the magnetic heads used for recording and reproducing signals to/from the magnetic disk are required to be positioned with very high precision. Moreover, the effective capacity of high-capacity magnetic disk cartridges can be increased by enabling multiple magnetic disk cartridges to be loaded and unloaded automatically instead of manually. Strong demand has therefore arisen for autoloading capability.
The magnetic disk cartridge of this type has a magnetic head access opening formed in one side surface of its cassette shell to a size allowing access to both surfaces of the magnetic disk from the exterior. Magnetic heads for recording and reproducing are inserted through the magnetic head access opening and brought near the rapidly rotating magnetic disk to record/reproduce signals. The cassette shell therefore has to be made to a thickness enabling insertion of the magnetic heads.
On the other hand, since the magnetic disk rapidly rotated inside the cassette shell is made of a flexible material, it has to be prevented from flopping during rotation and the space inside the cassette shell has to be precisely formed to suitable dimensions (generally to a thickness equal to that of the magnetic disk and the liners plus some amount of clearance). The required size can be achieved with high precision only if the cassette shell has sufficient rigidity, while sufficient rigidity can be obtained only if the cassette shell has sufficient thickness.
The thickness of the cassette shell therefore has to be made twice or more than that of the conventional 3.5-inch floppy disk having a thickness of about 3 mm. Accordingly, the wall thickness of the pair of cassette shell halves mated and joined to form the conventional cassette shell have also required a fairly large thickness.
However, a cassette shell with such a thick-walled structure has the drawbacks of being heavy and high in production cost owing to the large amount of material and long molding time required. Moreover, deformations known as sink marks are liable to occur in the thick-wall portions and give rise to degraded dimensional precision and poor appearance. These are quality problems that cannot be ignored.